Absorption spectroscopy has long been used for detecting the composition of matter and measuring trace constituents of matter. For example, photoacoustic spectroscopy can be used for ammonia detection. In an exemplary spectrometer, a CO2 laser with a resonant photoacoustic cell can be used to achieve a detection limit of 0.1 ppb for ammonia. Sensors have also been developed for ammonia detection in semiconductor industry as well as for atmospheric pollution monitoring. Somewhat relatedly, intra and inter pulse techniques with a long path length Herriot cell can be used to achieve ppb-level of detection of, for example, acrolein and acrylonitrile with a total integration time of approximately 10 seconds. A sensor can also be developed based on direct absorption in a multi-pass Herriot cell to monitor ethylene concentrations in vehicle exhaust and in air sampled from high-traffic urban tunnel.
Cavity ring-down spectroscopy (CRDS) is a form of absorption spectroscopy. CRDS utilizes the mean lifetime of photons in a high-finesse optical resonator with an absorbing medium present in the cavity. CRDS can excel in the low-absorbance regime where conventional methods have inadequate sensitivity.